1. Field of the Invention
The invention relates to a laser distance measuring system, more particularly to a laser distance measuring system with a shutter mechanism.
2. Description of the Related Art
U.S. Patent Application Publication Number 2001/0013929A1 discloses a device for optoelectronic distance measurement that includes two transmitters and two receivers. Two emitted light beams are respectively reflected internally and externally before being received by the receivers, respectively, thereby resulting in two measurement values. The difference between the two measurement values is then processed using analgorithm to result in a measured distance value. However, the device disclosed in this publication utilizes a large number of components, which results in complicated mechanical and circuit constructions, higher manufacturing and material costs, and a larger size. Moreover, there are inherent differences between the two transmitters and between the two receivers. Even if the two transmitters or the two receivers come from the same batch fabricated by the same manufacturer, there is no guarantee that the two transmitters or the two receivers will have identical characteristics. Therefore, measurement errors are unavoidable in the device disclosed in this publication.
U.S. Patent Application Publication Number 2004/0105087A1 discloses a laser distance measuring device that includes a transmitter and two receivers. An emitted light beam is split by a beam splitter to result in a reference light beam and a measurement light beam to be reflected by an object. The reflected measurement light beam and the reference light beam are received by the receivers, respectively, to result in two measurement values. The difference between the two measurement values is then processed using an algorithm to result in a measured distance value. However, although the measurement light beam and the reference light beam come from the same transmitter, measurement errors are still unavoidable since the measuring device includes two receivers. In particular, aside from the fact that there are inherent differences between the two receivers, one of the receivers receives the reference light beam under more ideal conditions, while the other receiver receives the reflected measurement light beam that is weaker and that has been subjected to some interference. Therefore, a bias voltage for the other receiver that receives the reflected measurement light beam is required to be larger to achieve higher gain for amplification. The difference between the bias voltages required by the two receivers is also a source of measurement error.
FIG. 1 shows a device for distance measurement that is disclosed in U.S. Patent Application Publication Number 2005/0083512A1. FIG. 2 shows an electro-optical distance measuring system that is disclosed in U.S. Pat. No. 6,781,675B2. Each of the device of FIG. 1 and the system of FIG. 2 includes a transmitter 101 and a receiver 102. Two shutter mechanisms 11 are further included in the device of FIG. 1. Through switching of the two shutter mechanisms 11 between two positions, a measurement beam for external reflection and a reference beam for internal reflection can be generated from the same light source for use in obtaining a measured distance value. However, the structure and operation of the device of FIG. 1 are difficult to realize since a change in the optical path of the reference beam can occur if the positions of the two shutter mechanisms 11 are imprecise, which can affect measurement accuracy. Moreover, the two shutter mechanisms 11 and the relevant driving mechanisms for the same will result in the disadvantage of a larger size. On the other hand, in the system of FIG. 2, the reference beam is obtained from a reflecting surface 12. In theory, the optical path of the reference beam is shorter and that of the measurement beam is longer such that the reference beam and the reflected measurement beam are received by the receiver 102 at earlier and later time points, respectively. However, in view of the response speed of the receiver 102 and the transient response of subsequent circuits, and in view of the fact that the speed of light is very fast, the late-arriving signal is likely to be affected by the early-arriving signal and is thus distorted, which leads to inaccurate measurement, especially when measuring short distances.